Recently, in the art of projectors, research and development efforts are being actively made to replace conventional discharge lamps with solid-state light sources such as LEDs (Light-Emitting Diodes) for better performance, smaller size, and lower cost. Rear projection TV sets and pocket projectors using LEDs as their light sources have already been in practical use.
In addition to LEDs, laser beam sources are also promising as solid-state light sources for use in projectors. Although laser beam sources undisputedly have a high potential as a light source, no projectors with laser beam sources have been in use yet because inexpensive G-color semiconductor lasers are not available for practical use and various limitations are required in view of the danger of laser beams.
Particularly, beam-scan projectors for displaying images by horizontally and vertically scanning a laser beam with an MEMS (Micro-ElectroMechanical Systems) scanner have to meet safety standards such as International laser safety standards IEC 60825 although they can be designed in a much smaller size than the existing projectors.
According to Class 1 and Class 2 (safe illumination intensity levels at which the laser beam can directly enter the eye) classified under IEC 60825, the laser beam source is required to limit its output laser power to a low level. For this reason, it is difficult for the beam-scan projectors to realize a practical sufficient brightness level, i.e. as high as the brightness level of conventional projectors with discharge lamps.
There are also known projectors of the type wherein a laser beam is not directly scanned, but is applied to a liquid crystal light valve, a DMD (Digital Micromirror Device), or a two-dimensional optical modulator such as an LCos (Liquid Crystal on Silicon), and an image generated by modulating the laser beam with the optical modulator is projected at an enlarged scale by an optical system (projection lens, etc.). The projectors of this type are considered to be of higher luminance than the beam-scan projectors.
Generally, such a projector has a single optical modulator or three optical modulators for the colors R, G, B. Projectors with a single optical modulator are lower in cost. If a single optical modulator is employed, then laser beams in the colors R, G, B are time-multiplexed and applied to the optical modulator to produce color images.
As regards lasers for use as light sources, semiconductor lasers in the colors R, B are available for practical use, but semiconductor lasers in the color G are not yet available for practical use. Although SHG (Second Harmonic Generation) lasers combined with a nonlinear crystal for wavelength conversion are available as G-color lasers, the SHG lasers are highly expensive.
There is known, as a projector with an optical modulator, a projector of the type wherein images that are projected from respective projection engines are tiled into a single image (see Patent document 1). If the projection engines incorporate respective G-color laser beam sources, then the projector of this type is highly expensive.
As described above, the projectors with optical modulators are problematic in that it is difficult to achieve both a lower cost and a higher resolution.    Patent document 1: JP No. 2008-039985A